A fluororesin film can maintain weather resistance, transparency, stain resistance, etc. for a long period of time, and therefore such a film is used as a covering film for membrane structures (such as green houses, sports facilities or tents) or a surface laminating film for various substrates.
On the back side surface of a fluororesin film to be used for a covering film, a coating film (a printing layer) comprising a printing ink is likely to be formed for the purpose of e.g. imparting visual quality. Further, on the back side surface of a fluororesin film to be used for a surface laminating film, a coating film (an adhesive layer) comprising an adhesive is formed for adhering the surface laminating film to a substrate.
However, if the fluororesin film comprising such a coating film is used outside for a long period of time, there is a problem that materials (e.g. a resin or colorant (such as a pigment or a dye)) for the coating film are deteriorated due to ultraviolet light, that is, a problem of the weather resistance of the coating film.
As a fluororesin film having a coating film with improved weather resistance, the following are proposed:
(1) A fluororesin film having a printing layer comprising a fluororesin composition, formed on at least one side of a fluororesin film body (Patent Document 1)
(2) A fluororesin film having a printing layer or an adhesive layer present on the back side surface of a fluororesin film body containing titanium oxide particles or zinc oxide particles (Patent Document 2)
(3) A fluororesin film having a layer comprising a cured product of a curable fluororesin containing a ultraviolet absorber, laminated on a surface-treated surface of a fluororesin film body (Patent Document 3)
(4) A fluororesin film comprising a fluororesin film body, and a coating film made of a coating film-forming composition containing a fluororesin having a specific functional group, formed on the fluororesin film body (Patent Document 4)
However, the fluororesin films (1) and (4) have the following problems.
(i) As a resin, only a specific fluororesin can be selected, that is, the type of a material for a coating film is restricted.
(ii) In the case of imparting weather resistance also to a colorant, only a specific inorganic pigment (a specific color) having a weather resistance can be selected as a colorant, that is, the material for a coating film is restricted.
Further, the fluororesin film (2) has the following problems.
(i) The dispersibility of titanium oxide particles or zinc oxide particles in a fluororesin is poor, and such particles are agglomerated and a fluororesin film body thus whitened, whereby transparency is deteriorated.
(ii) The fluororesin film body is deteriorated by photocatalysis of the titanium oxide particles.
(iii) Zinc oxide particles are reacted with fluorinated compounds liberated from a fluororesin at the time of producing a fluororesin film or using it outside, whereby they are formed into zinc fluoride, and therefore the ultraviolet shielding performance is weakened.
As a result, the coating film is deteriorated due to ultraviolet light.
Further, the fluororesin film (3) has the following problems.
(i) As a resin, only a specific fluororesin can be selected, that is, the type of a material for a coating film is restricted.
(ii) Since a ultraviolet absorber is easily decomposed or bled out, the lifetime of the ultraviolet absorber is short as compared with the lifetime of a fluororesin.
(iii) Since the surface of a coating film on the side irradiated with the ultraviolet light is not sufficiently shielded from the ultraviolet light, the surface easily deteriorates.